The present invention relates to ceramic fibers used as reinforcement in fabricating composite material.
Ceramic fibers present mechanical properties, such as breaking stress and Weibull modulus, that are stronger than those of the corresponding monolithic ceramics, mainly because of the small volume that is stressed and a population of flaws that is small.
Nevertheless, the mechanical properties of ceramic fibers remain limited by the presence of flaws associated with the techniques used for preparing and/or handling them (fiber drawing, pyrolysis, sintering, friction, etc). Although these faults are essentially localized at the surfaces of the fibers, they have a significant impact on their mechanical performance. So far as a fiber is concerned, the presence of such flaws gives rise to limited breaking stress, a relatively low Weibull modulus, and a lifetime in air, under load, and at constant moderate temperature that is too short for applications such as hot parts in aeroengines.
Document U.S. Pat. No. 6,579,833 describes a method enabling a carbon coating to be formed by synthesis on the surfaces of metallic carbides such as silicon carbide (SiC). Document WO 2005/007566 describes an application of that method to forming microporous carbon with controlled porosity. The method implements reactive heat treatment based on halogen gas generating a layer of microporous carbon at the surface of the carbon.
That method has been applied to ceramic fibers (ZMI, Hi-Nicalon®, Sylramic®, Tyranno® fibers) as described in the documents “Carbide Derived Carbon (CDC) Coatings for Tyranno ZMI SiC Fibers”, Y. Gogotsi et al., Ceram. Eng. & Sci. Proc., 24, pp. 57-62 (2003) and “Mechanical Properties of Carbon and BN Coated SiC Fibers”, Y. Gogotsi et al., Ceram. Eng. & Sci. Proc., 24, pp. 225-230 (2003) where the formation of a carbon layer at the surface of the fiber is observed, but without any improvement in its mechanical properties (monotonic traction at ambient temperature).
Applying heat treatment to ceramic fibers with a halogen gas is also used in document WO 2005/092610 as an intermediate step in a method of making a coating on ceramic fibers of the boron-aluminum-nitrogen (BAN) type, e.g. formed by a mixture of BN and Al(O)N, in order to improve the ability of ceramic composite to withstand oxidation.
Although ceramic fibers provided with such a coating present improved mechanical properties at ambient temperature, their ability to withstand oxidation and their lifetime remain insufficient.